Chemistry ATOMIC, PHYSICAL & CHEMICAL PROPERTIES

General Properties :

`ast` HALO : Sea Salts: Generally they are called halogen because they are sea salts forming elements.

`ast` Important point : `At` (astatine) is a radioactive element.

`ast` General Properties :

`ast` State and Nature :

(a) `F` and `Cl` are Gas (b) Br : Liquid

(c) `I`, `At` is solid (d) All are non metallic

(e) Metallic character down the group `(F -> I)` non metallic character decreases

(f) `I` has metallic lustre on heating and it shows sublimation property.

`ast` `text(Atomic Radii, Ionic Radii, Boiling point and Melting point, Density :)` All these character increases down the group

`ast` `text(Ionisation potential and Electronegativity :)` Down the group atomic size increases so `IP` and `EN` decreases down the group

`ast` `text(Electron Affinity (E.A.):)` `Cl > F > Br > I`. Generally down the group electron affinity decreases but the electron affinity of `Cl` is
more than `F` because due to the small size and high electron density of `F` the incoming electron is not easily enters as compared to `Cl` because it's size not as small as size of `F` and not large as `Br` and `I`.

`text(Colour Properties :)`

`F` : Pale yellow. `Cl` : Greenish yellow `Br` : Red `I` : Voilet purple.

Outermost electronic configuration is : `ns^2 np^5` due to the presence of unpaired electron they absorb visible light and reflect complementary light and exhibit colour.

`F` absorbs violet colour light and appears yellow. It will absorb yellow coloured light and appears violet.

(d) Astatine being stable could have absorbed orange or red light and would have exhibited indigo or bluish colour.

`ast` `text(Valency and Oxidation state :)`

(a) `ns^2np^5`

(b) valency = 1

If Halogens combines with more `E.N.` elements then `O.S. = +1`

If Halogens combines with more `E.N.` elements then `O.S. = +1`

(c) For `Cl`, Ground State : See fig.1.

`ast` `text(Bond Energy :)`

Bond Energy :

`F-F` bond dissociation energy is less than that of `Cl - Cl` and `Br - Br`. It is due to larger inter electronic (electron- electron) repulsion between the non bonding electrons in the `2p` orbitals of fluorine atom, then these in the `3p` orbitals of chlorine atoms.

`star` `text(Oxidising Power :)` The electron affinity, or tendency to gain electrons reaches a maximum at chlorine. Oxidation may be regarded as the removal of electron so that an oxidising agent gains electrons. Thus the halogens act as oxidi zing agents. The strength of an oxidising agent (i.e. oxidation potential) depends upon several energy terms and represented by following diagram.

`X_2 overset(DeltaH)-> 2X underset[(Delta H)_(eg)] overset[(+)2e^(-)]-> 2X^(-) overset(Delta H)-> `hydrated ion

`Delta H` (reduction potential) `= Delta H` (dissociation energy) `+ Delta H` (electron gain enthalpy) `+ Delta H`

See fig.2.

Thus oxidising powers decrease on descending in group `VII`. Fluorine is so strong oxidising agent that H oxidizes water to oxygen. The oxidation of `H_2O` by `Cl_2` is thermodynamically possible but since the energy of activation is high this reaction does not occur.

`F_2 + H_2O -> 2H^(+) + 2F^(-) +1/2 O_2`

`Cl_2 +H_2O -> HCl + HOCl`

Iodine is even weaker oxidising agent and the free energy change indicate that energy would have to be supplied to make it oxidise water.

Chemical Properties :

Reaction with `H_2` : All halogens reacts with `H` to form hydrogen halides.

(a) `F_2 + H_2 oversettext(Dark)-> H_2F_2`

`Cl_2 + H_2 oversettext(Light)-> 2HCl`

`Br_2 + H_2 overset(Delta)-> 2HBr`

`I_2 + H_2 overset(Delta)-> 2HI`

(b) Reactivity of Halogens `F > Cl > Br > I`

(c) `HCl` in gaseous state is hydrogen chloride. While `HCl` in aqueous solution is hydrochloric acid.

(d) `HCl`, `HBr`, `HI` acts as reducing agent.

(e) `H_2F_2` cannot decompose into `H_2` and `F_2` and so it can never act as reducing agent.

(f) `H_2F_2` has highest boiling point and exist in liquid state.

(g) `HCl`, `HBr` and `HI` exist on gaseous state.

(h) `HCl` b.p. is very low and after `HCl` b.p. increases because Van der waals forces increases (`uparrow`) and so b.p. (`uparrow`).

`ast` `text(Reaction With)` `H_2O :`

(a) `H_2O` acts as reducing Agent only with `F`.

`2F_2 + 2H_2O -> 2H_2F_2 + O_2`

(b) `Cl_2 + H_2O -> HCl + HClO` Hypochlorous acid `oversettext(Light) -> HCl +[O]`

So `Cl_2` in presence of moisture acts as bleaching agent.

(c) `H_2O + I_2 ->` No Reaction.

(d) All these halogen are soluble in water except `I_2` when `KI` solution is added `I_2` will also dissolve because adsorption of `I_2` molecule on the surface of iodide ion `KI_3` is formed which is a highly ionic compound.

`ast` `text(Displacement Reaction :)`

(a) `F_2 + 2NaX -> 2NaF + X_2` {`X = Cl, Br, I`}

`Cl_2 + 2NaX -> 2NaCl + X_2` {`X = Br_2, I_2`}

So order of displacement `[F_2 > Cl_2 > Br_2 > I_2]`

(b) If Halogen is in `-ve` O.S. then it is replaced by more `E.N.` element.

(c) If Halogen is in `+ve` O.S. then it is replaced by less E.N. element.

`ast` `text(Reaction with Metals :)` Metal halides are formed `F > Cl > Br > I`

`ast` `text(Reaction with Non Metals :)` Non metallic halides are formed. Eg. `NF_3, PCl_3` etc.

`ast` `text(Reaction with) NH_3 :`

(a) `3F_2 +NH_3 -> NF_3 + 3HF`

(b) `3Cl_2 + NH_3 -> NCl_3 + 3HCl`

(c) `I_2` is less reactive and so remains same and Ammoniated compound will form.

`3I_2 + 2NH_3 -> NI_3 . NH_3 + 3HI`

`ast` `text(Reaction with NaOH :)`

(a) `F_2 + NaOH (dil.) -> 2NaF + OF_2 + H_2O`

`2F_2 + 4NaOH (conc.) -> 4NaF + 2H_2O + O_2`.

They are both redox reactions.

`Cl_2 + NaOH -> NaCl + NaClO_3 + H_2O`

`Br_2 + NaOH -> NaBr + NaBrO_3 + H_2O`

Formation of Oxides : No oxides of `F` because of its maximum E.N. character. See fig.1.

(a) All these oxides are acidic

(b) Acidity decreases down the group and maximum active oxide is `Cl_2O_7`

`ast` `text(Formation of Oxyacids :)`

(a) `F` does not form any oxyacid because High E.N.

(b) Rest oxyacids are : See fig.2.

(c) All these oxy acids are acidic

(d) Acidity and thermal stability decreases down the group and maximum acidity and thermal stability will be of `HClO_4`

(e) Thermal stability of acids increases with the increasing O.S. of the halogens or with increase in the no . of oxygen atom.

(f) Oxidising Power :

(i) All these oxyacids acts as strong oxidizing agents.

(ii) Stability of anions increases from `ClO^(-)` to `ClO^(4-)`, the oxidising power decreases from `ClO` to `ClO_4`

(iii) Order of oxidising agent `HClO > HClO_2 > HClO_3 > HClO_4`

`ast` `text(Miscellaneous Reaction :)`

(a) `I_2 + Na_2S_2O_3 text((Hypo)) -> 2NaI text((Colourless)) + Na_2S_4O_6`

This reaction is employed for identification of `I_2`

(b) Dry `Cl_2 + Ca(OH)_2(s) -> CaOCl_2` (Bleaching Powder) `+ H_2O`

`ast` `text(Inter Halogens :)` Product obtained by uniting two halogen compounds : See fig.3.

These inter halogens have polarity because of different E.N.

`text(Note : Abnormal Behaviour of Fluorine as Compared to Other Members :)`

Fluorine differs considerably from other halogens due to-

(i) Small size

(ii) High electronegarivity.

(iii) Non availability of `d`-orbitals in its valency shell.

(iv) Low bond dissociation energy of `F-F` bond.

(a) Boiling point of `HF` is the highest and for other increases down the group.

See fig.4.

(b) Due to hydrogen bonding `HF` is a liquid while `HCl`, `HBr` and `HI` are gases.

(c) Fluorine, being the most electronegative gives `SF_6` while other member do not form hexahalides with sulphur.

(d) It exhibits oxidation state of only `-1`.

(e) It is the strongest oxidising agent.

(f) It liberates oxygen as well as ozone with water.

(g) `HF` does not ionize while `HCl`, `HBr` and `HF` ionize in aqueous solution.

(h) Solubility of salts :

(i) `AgF` is soluble in water while `AgCl`, `AgBr` and `AgI` are insoluble.

(j) It combines with hydrogen with explosion at a low temperature and even in the dark. No other halogens combines so readily.

`H_2 + F_2 -> 2HF`

(k) It liberates oxygen as well as ozone with water.

`2H_2O + 2F_2 -> 4HF + O_2`

`3H_2O + 3F_2 -> 6HF + O_2`